1. Field of the Invention
The present invention relates to an interferential measurement device for measuring the relative position of objects in at least one direction of measurement, and more particularly, to a measurement device where by means of light beam diffraction a material measure is scanned by a scanning unit having at least one light source.
2. Description of the Prior Art
Interferential measurement devices are used particularly in a processing tool for measuring the relative position of a tool with respect to a workpiece to be processed.
For example, the dissertation of Fromund Hock, Stuttgart, 1978, entitled "Photoelektrische Messung der Anderung von Langen und Winkelpositionen mit Hilfe von Beugungsgittern," [Photoelectric Measurement of the Change of Lengths and Angular Positions with the Aid of Diffraction Grids], Section 11.6, describes an interferential measurement device for two directions of measurement. A collimated light beam bundle emanating from a light source passes through a cross grid of a scale whereupon the beam bundle is diffracted into diffraction beams deflected into the first and second directions of measurement. The cross grid of the scale extends diagonally to the two directions of measurement. The beam bundle then passes through four spatial frequency filter screens and is displayed by means of two lenses on a first linear grid. The first linear grid is oriented in the first direction of measurement and unites the diffraction beams which have been deflected at the scale into this first direction of measurement. The combined diffraction beams of the first direction of measurement are guided by means of a third lens to first detectors, which provide scanning signals for the generation of measured values for the first direction of measurement. The diffraction beams which have been deflected at the scale into the second direction of measurement are displayed with the aid of a fourth lens on a second linear grid which is oriented in the second direction of measurement. The second linear grid unites the diffraction beams which have been deflected at the scale into this second direction of measurement. The combined diffraction beams of the second direction of measurement are guided to second detectors which provide scanning signals for the generation of measured values for the second direction of measurement. This measurement device has the disadvantage of having a costly construction due to the many pieces of hardware required.
An interferential measurement device for one direction of measurement is known from European Patent Application 0 163 362 B1. A light beam bundle emanating from a light source and collimated by means of a collimator lens is split into a plurality of diffraction beams in the course of a first passage through a scanning grid and is again diffracted under reflection at a scale grid. The reflected diffraction beams once again pass through the scanning grid where they are yet again diffracted and brought into interference. The interfering diffraction beams in the direction of resultant 0th, +1st and -1st ordered beams are separated by a collimator lens and directed to three detectors for the generation of electrical scanning signals to obtain measured values.
In this measurement device the scale grid must be embodied in such a way that the diffraction beam of the 0th order is suppressed at the scale grid. This results in the disadvantage that diffraction beams of the 3rd order appear at the scale grid so that the scanning signals obtained by means of the detectors have a high harmonic wave component which does not permit a high interpolation of the scanning signals. Since in practice the suppression of the diffraction beam of the 0th order at the scale grid is only possible within certain tolerances, with a short distance between the scanning grid and the scale grid, an interfering sub-harmonic appears in the scanning signals which also does not permit high interpolation of the scanning signals. The direct reflections of the scanning grid which strike the detectors by lowering the modulation degree have shown to be a further disadvantage.
It is an object of the present invention to provide an interferential measurement device for at least one direction of measurement of considerably higher measurement accuracy and simpler structure.
The advantages obtained by means of the invention consist in particular in that by the use of a cross grid only, diffraction beams are detected which are deflected at the cross grid in the direction of measurement as well as vertically to the direction of measurement. Thus the diffraction beam of the 0th order of the cross grid as well as the beams directly reflected by the scanning grid do not reach the detectors. Therefore the cross grid can be embodied in such a way that diffraction beams of the 3rd order are suppressed at the cross grid thereby obtaining scanning signals having a higher degree of modulation and an extremely small harmonic wave component. Moreover, diffraction beams of even order of the scale, such as appear in the measurement device of European Patent Application 0 163 362 B1 in case of incomplete formation of the scale grid, do not fall on the detectors. The signal shape improved by this permits high interpolation of the scanning signals. Furthermore, with a short distance between the scanning grid and the cross grid no interfering sub-harmonics are created in the scanning signals.